The present invention relates to electromagnetic interference (EMI) shielding, and more specifically, to board level shielding of electromagnetic interference (EMI) generated through various components mounted on a printed circuit board (PCB).
An electromagnetic signal is propagated with an electrical field component and a magnetic field component. The electromagnetic signal causes circuit components to radiate a portion of the spectral energy of the propagating signal, resulting in electromagnetic interference. Electromagnetic interference is the generation of undesired electrical signals in electronic system circuitry because of the unintentional coupling of impinging electromagnetic field energy.
Enormous progress in nanotechnology has made an electronic system smaller and has increased the density of electrical components within an instrument. In highly compact electronic systems, the dimension of the average circuit element is decreasing which in turn favors the radiation of higher frequency signals. The increasing operating frequency in these electrical systems results in an elevated level of high frequency electromagnetic interference (EMI). The prevalence of high frequency systems and portable electronic circuitries are creating a very complex spectral environment for the operation of sensitive electrical/electronic systems. Accordingly, it is often advantageous to shield an electrical/electronic component to prevent it from emitting EMI, or to shield an electrical/electronic component from EMI emitted elsewhere.
The electromagnetic interference shielding of electronic component assemblies has taken many forms. Sensitive or radiating devices may be covered with a lid and/or an enclosure which is connected to ground potential in the process of securing the cover in place. Shielding close to the source, where the field intensity is the highest, requires greater shield efficiency to contain the field.
It is common to shield the sensitive, electromagnetic interference receiving component or even the entire circuit board. Polymer thick film conductor materials, such as a screen-printable copper filled epoxy paste, are generally used to form a shield.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other “target” devices from other source devices. Such shielding is provided as a barrier which is interposed between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device, or as a “can” which covers a discrete component or components of the device. The housing or can may be formed of a metal such as steel, aluminum, or magnesium, or alternatively, of a plastic or other polymeric material which is filled to form and electrically-conductive material, or which may be provided with a conductive coating generally applied across the interior surfaces of the housing.
The coating may be an electrically-conductive paint, a conductively-filled, molded elastomeric layer, a metal foil laminate or transfer, or a flame-sprayed or other deposited metal layer. A conductive gasket may be used to provide electrical continuity between the coating-layers applied to the various mating housing parts.
Various enclosed systems are known to a person of ordinary skill in the art. The enclosed systems are powered by external alternating current and are shielded from electromagnetic interference by the incorporation of internal shields which are connected to a ground potential. A metal cabinet housing which encloses the system may be designed to function as a shield. However, metal housings are often expensive, heavy, and difficult to make in complex shapes. The inside of a molded plastic housing may be coated with a thin metal film through vacuum metallization, but this process often yields a brittle less flexible shield. Another method is to coat the enclosure with a thin film of a conductor using a metal-filled paint. A metal-filled plastic may also be used to form the housing.
U.S. Pat. No. 4,739,453, issued to Kurokawa, discloses a shielding apparatus for shielding electric circuitry mounted on a printed circuit board against interfering electric waves. The shielding apparatus comprises a multi-layer printed circuit board having a circuit-trace printed on it. A circuitry block is mounted on the multi-layer printed circuit board and is electrically connected to the circuit-trace so that the circuitry block is connected to one other circuitry block mounted on the printed circuit board via the circuit-trace. A metallic plate is provided in order to cover and isolate one circuitry block from the other circuitry block.
U.S. Pat. No. 5,717,577, issued to Mendolia et al., discloses an apparatus for shielding electromagnetic emissions created by electronic components and circuitry. The apparatus comprises a printed circuit board on which the electronic components are mounted, a conductive gasket, and a metal shield can for shielding electronic components. The printed circuit board includes a ground plane and a ground pad ring located on a top surface of the printed circuit board surrounding the electronic components. The conductive gasket is positioned between the shield can and the ground pad ring to provide a conductive seal and to attenuate electromagnetic emissions within the shield can.
U.S. Pat. No. 5,763,824, issued to King et al., discloses a shielding cover in combination with an electrical assembly. The electrical assembly has an electrical ground, an electronic component electrically connected to the assembly, and a conductive frame which is disposed about the electronic component. The conductive frame has a mounting surface and is electrically connected to the ground. The shielding cover comprises a lid and an electrically conductive adhesive disposed between the conductive frame and the lid.
U.S. Pat. No. 6,016,083, issued to Satoh, discloses an electronic circuit apparatus for suppressing electromagnetic radiation. The apparatus comprises an electronic circuit mounted on a top surface of a printed-circuit board, an input-output terminal mounted on the printed-circuit board and a shield-case for suppressing electromagnetic radiation from the electronic circuit. The described apparatus is further provided with a conductor structure, positioned between the electronic circuit and the input-output terminal, connecting a first ground-pattern attached to a bottom surface of the printed-circuit with the shield-case.
The existing shield assemblies provide limited reuse capabilities and are limited to single compartments. The materials such as paint or plated injection molds or thermoformed materials have not deployed successfully deployed due to low shielding capabilities. A form-in place gasket has been tried in the existing assemblies, but not in conjunction with a thermally sprayed coating and an electrically conductive plastic.
It is desirable to provide shielding to small board level components in order to reduce emissions and to protect against electromagnetic interference. Preferably, the board level shield is attached to a printed circuit board (PCB) by using a mechanical means, and is rigid enough to protect printed circuit board (PCB) components. The disclosed board shield can provide the desired shielding effects as per the specifications for proper operation for the circuitries.
Therefore, there is a need for a board level shielding device which can be easily installed and available in flexible shapes for adjusting in multiple cavities. The device should be capable of being easily attached to the circuit to provide effective shielding. The present invention also provides high quality, low-cost applications shield assemblies for thermal management and EMI/RFI shielding.
Accordingly, it is an object of the present invention to provide an apparatus for board level shielding of electromagnetic interference (EMI) generated in electronic circuitry.
It is another object of the present invention to provide a method for assembling an apparatus for board level shielding of electromagnetic interference (EMI).